Four months into living in this house I had nothing better to do one weekend and finally got round to tackling the long-term state of the loo.
The first stage was bleach, in this case Sainsbury's basics at 28p for a big bottle. This had impressive results and got rid of most of the discolouration.
However, what was left was the limescale or whatever it is. More bleach and scrubbing with the loo brush eventually sorted this out.
There was still discolouration/limescale deep down in the loo at the back, which I could have ignored but I'm a perfectionist. I tried scrubbing it with those wire balls and scouring pads (using the loo brush to push them around) but that wasn't successful. I finally resorted to hacking it all off with the end of a screwdriver, following the slow process of fishing out all the bits, which didn't want to be flushed away! It doesn't sound very professional but I got there in the end.
I'll try the vinegar suggestion next time.